1. Field of the Invention
The present invention generally relates to liquid crystal display devices and, more particularly, to a liquid crystal display device equipped with a backlight device that illuminates a liquid display part.
2. Description of the Related Art
First, a description will be given, with reference to FIG. 1, of a conventional liquid crystal display device. FIG. 1 is an exploded perspective view of a conventional liquid crystal display device.
A liquid crystal panel 1 of the liquid crystal display device, which displays information thereon, is accommodated in a housing 2 that also serves as a decorative board. A backlight device is provided under the liquid crystal panel 1 so as to make the liquid crystal display legible by illuminating from backside.
The backlight device has light sources 3, a light-guiding plate 4 and optical sheets 5a and 5b. The light sources 3, the light-guiding plate 4 and the optical sheets 5a and 5b are accommodated in a backlight housing comprising upper and lower housings 6a and 6b. 
A light emitted from each of the light sources 3 propagates inside the light-guiding plate 4, and exits from a front surface of the light-guiding plate 4 toward the liquid crystal panel 1. A reflective panel 7 is provided on a side of the light-guiding plate 4 opposite to the light-guiding plate 4, and the light projected from each of the light sources 3 and incident on the light-guiding plate 4 exits only in a direction toward the liquid crystal panel 1.
The light that exits from the light-guiding plate 4 is irradiated onto the liquid crystal panel 1 after being subjected to a predetermined optical process such as diffusion or convergence by the optical sheets 5a and 5b. Thereby, the background of the liquid crystal panel 1 becomes bright moderately, which makes the display on the liquid crystal panel 1 legible.
Although the two optical sheets 5a and 5b are used in the device shown in FIG. 1, a single optical sheet may be used if a desired backlight effect can be obtained, or there may be a case where more than three optical sheets are used. Additionally, although the two light sources 3 are provided on opposite sides of the light-guiding plate 4 in FIG. 1, only one light source may be provided on one side.
In the structure of the backlight device shown in FIG. 1, each of the optical sheets 5a and 5b has the front surface and the back surface, and does not function correctly if it is mistakenly incorporated in a wrong direction. Therefore, it is necessary to check, after assembling the backlight device, whether the optical sheets 5a and 5b have been incorporated and whether their front and back surfaces face correctly.
However, after assembling the backlight device, the optical sheets 5a and 5b are covered by the liquid crystal panel 1 and the backlight housing 6a. Therefore, there is a problem in that it is difficult to check visually from outside, after the assembly of the backlight device, whether the optical sheets 5a and 5b are appropriately incorporated.
Here, if an ambient temperature of the liquid crystal display device is raised in an environmental test etc., the optical sheets 5a and 5b will expand thermally. Under such circumstances, there is a case in which a periphery of each of the optical sheets 5a and 5b shifts toward the center thereof without extending outwardly. In such a case, each of the optical sheets 5a and 5b deforms into a fine-wavy form as shown in FIG. 2B.
That is, if the gap between the backlight housing 6b and the light-guiding plate 4 is large, each of the optical sheets 5a and 5b makes a smooth deformation in which a center section protrudes as shown in FIG. 2A. However, if the gap is small as shown in FIG. 2B, each of the optical sheets 5a and 5b will deform into the wavy form having many fine waves. If the optical sheets 5a and 5b deform as shown in FIG. 2B, there is a problem in that a light passing through the optical sheets 5a and 5b is influenced and unevenness occurs in the backlight illumination.
Moreover, although a light-guiding plate 4 also expands in connection with the temperature rise, the light-guiding plate 4 deforms so that the center portion thereof is bent since the periphery thereof is fixed and the thickness thereof is larger than the thickness of the optical sheet and the light-guiding plate 4 has rigidity. Here, when the center portion of the light-guiding plate 4 bends in a direction to separate from the liquid crystal panel 1, the gap between the liquid crystal panel 1 (the upper backlight housing 6b) and the light-guiding plate 4 expands only in the center portion. Therefore, the space within which the optical sheets 5a and 5b can deform is expanded, and there is a problem in that a magnitude of deformation further increases.
Each of the light sources 3 shown in FIG. 1 consists of a fluorescence tube, which generally uses an ultraviolet radiation of mercury. FIG. 3 is a perspective view of the light source 3 that consists of a fluorescence tube. In the light source 3, opposite ends of a fluorescence tube 3a is attached to fluorescence tube support members 3b, and a reflector 3c is provided around the fluorescence tube 3a. The reflector 3c has a function to reflect a light emitted from the fluorescence tube 3a and converge the reflected light onto an incident light end surface of the light-guiding plate 4. The fluorescence tube 3a also emits heat when emitting a light. Such a heat is released through the reflector 3c and the fluorescence tube support members 3b. 
As mentioned above, since an ultraviolet radiation of mercury is used for the fluorescence tubing 3c, mercury vapor is enclosed within a glass tube, which constitutes the luminescence portion. Here, if a wall-surface temperature of the glass tube changes, a mercury vapor pressure inside the glass tube changes, which results in a change in the luminous efficiency. Such a change in the luminous efficiency takes a peak value (maximum) at a certain temperature if the glass wall surface. Therefore, in order to maintain a high luminous efficiency, it is necessary to maintain the wall surface of glass tube at a constant temperature.
Moreover, a cold cathode tube can also be used for the fluorescence tube. In such a case, when a cold cathode tube emits electrons, much electric power (=cathode drop voltage×tube current) near the cathode. Such an electric power is reflected to as a reactive power, and most parts of the reactive power are converted into heat. If the liquid crystal display device is enlarged and the intensity of luminescence of the backlight is raised, a cathode drop electrical potential difference and a tube current, which are the main components of a tubing electrical potential difference, will go up inevitably. Consequently, generation of heat of the fluorescence tube end section, which is the cathode section, will become larger relative to other part.
As mentioned above, generation of heat of the fluorescence tube-end portion, which is the cathode section, increases, the temperature near the fluorescence tube-end portion rises and creep may occur in a solder connecting a terminal and a lead wire. If creep occurs in a solder, it causes a poor connection, and there is a problem in that a reliability of dependability of a connection falls remarkably. Generally, a creep phenomenon starts to occur at 0.5 times the melting point of the material. Usually, since the melting point of a solder is 183° C., a half of the melting point is 91.5° C. It is appreciated from the experiments that if the temperature of the solder exceeds 100° C., the creep appears remarkably.
Moreover, when the temperature near the fluorescence tube-end portion rises, there is a problem in that heat deformation and thermal degradation occur in a resin member such as the light-guiding plate 4 or a plastic frame, which are members arranged around the fluorescence tube. In order to solve the problem caused by the generation of heat in such a fluorescence tube and to maintain the luminous efficiency at a high value, it is necessary to properly control a heat radiation from the fluorescence tube.
However, in the structure of the conventional light source 3, a heat is radiated from only the by reflector 3c which merely encloses the fluorescence tube 3a and the fluorescence tube support member, and the temperature control of the fluorescence tube and a peripheral portion thereof according to the heat radiation is not taken into consideration. Therefore, there is a problem in that the luminous efficiency of the fluorescence tube cannot be maintained in a good state.